1. Field of the Invention
The present invention relates to a distance measuring device by which a distance between an optical apparatus such as a camera, and a subject is measured.
2. Description of the Related Art
As an active autofocusing distance measuring devices there is known a device in which infrared light is emitted from a camera to a subject and the reflected light from the subject is received by a linear line sensor provided in the devices so that the subject distance between the camera and the subject is sensed based on triangulation.
Namely, an image of light reflected from the subject is formed on the line sensor, and the position on the line sensor at which the image is formed changes in accordance with the subject distance. The line sensor outputs an electric current signal in accordance with the position of the image of the light, and the signal outputted from the line sensor is converted to a digital signal by an A/D converter, which converts the analog signal with a predetermined converting step to obtain the digital signal.
In this device, to improve the accuracy of the subject distance measurement, it is necessary that the position of the light spot on the line sensor should be sensed with a high accuracy. However, if the converting step is large, in other words, if the resolution of the A/D converter is low, the digital signal does not change when the analog signal is changed by a small amount and this causes an error in measuring the subject distance.
Conversely, if an A/D converter having a small converting step to give a high resolution is used in the distance measuring device, the current consumption of the device increases and the cost of the device also increases.